This is a common question asked by tons of students. Many argue that it's worthless, and will only tell you that you're not ready to take the test, which is OBVIOUS. Others argue that it does a poor job of highlighting your weaknesses. However, I firmly believe that you SHOULD take a diagnostic test, especially if you have access to an exam that you wouldn't otherwise take. Let me explain.... - Do NOT use a good AAMC FL as a diagnostic, because you're simply wasting a valuable practice test that you should've saved for later. - Use a FL that you were never going to do anyways, and it doesn't hurt to do it. It will put into realization the seriousness of the MCAT and how much dedication it will take to do well on the test. You will not regret taking the diagnostic exam. Do not pay much attention to the score, rather just take it to get a good feel for what you will be dealing with for the next 1-4 months. - It may also give you more of a timeline of how long you should be studying for.